Salt is one of the most abundant materials on earth and is one of the largest volume inorganic materials used in industry in the production of chlorine and caustic soda by the electrolytic processes. It is used in the manufacture of many products, both organic and inorganic. Salt is produced by direct removal as rock salt from underground deposits, by solution mining underground deposits, or by evaporation from solution mining or naturally occurring brines or sea water. Underground deposits are large beds of concentrated salt which have been deposited through evaporation of brines over the geological ages. Underground deposits are mined as rock salt using conventional mining techniques or by solution mining, creating a brine. In solution mining, water or unsaturated brine is pumped into the salt bed, the salt is dissolved in the water, and the resultant brine is brought to the surface. After a period of operation, determined by the size of the cavern produced and governed by cavern salt wall stability and surface subsidance considerations, the well is shut down, leaving an unused cavern. These caverns may be used for storage of petroleum reserves and chemicals among other fluid materials or the well might be plugged in accordance with regulations. Many processes and techniques have been disclosed for storage of fluid materials in these empty caverns.
The raw brine produced and recovered at the wellhead of an active solution mining well must be purified for many of its uses, especially if it is to be electrolyzed in chlor-alkali cells to make chlorine, sodium hydroxide and hydrogen or for producing sodium. Many processes and techniques have been disclosed for the purification of the brine produced by the solution mining processes in electrolytic cells. U.S. Pat. No. 3,655,333 is an example disclosing a process for purifying salt or brine already produced. U.S. Pat. No. 5,028,302 and 5,126,019 are examples of processes for purification of brine for use in chlor-alkali membrane cell plant installations which require strict specifications on the amount if impurities, including calcium, magnesium, strontium, sulfate and chlorate ions.
The purification of raw brine for use in chlor-alkali electrolytic cells may involve several purification operations depending on the type of electrolytic cell employed and the purity specifications which allow for the continuous and efficient operation of the cells. Primary treatment systems include means for treating brine with caustic soda (or cell liquor from the cathode compartment of the cell) and sodium carbonate or any other additives that may be necessary to eliminate impurities such as calcium, magnesium, iron, strontium, etc. An important specification for primary treated brine for use in chlor-alkali electrolysis cell is that it contain less than about 20 ppm divalent calcium and less than about 1 to 2 ppm divalent magnesium. The next purification operation usually includes polishing the primary treated brine to remove solids in a thickener followed by a sand filter or other type filter to remove line solids from the thickener overflow. Additional or secondary treatment systems may be employed in those cases where specifications for the brine to be electrolyzed are such that specific ions, such as calcium, magnesium, other divalent cations and sulfate anions in brine to be electrolyze in membrane type chlor-alkali electrolytic cells, must be reduced to a minimum. These purification operations have traditionally been carried out at the chlor-alkali electrolytic cell plant installation.
Processes have been described in the prior art for the production of purified brine at the salt mining site. One such process for the production of purified brine injects the purification chemicals into the water during the solution mining of the brine in an operating well and is described in U.S. Pat. No. 3,606,466 issued on Sep. 20, 1971 to Dudley P. Fernandes. This process is not practiced commercially because it does not produce brine of sufficient purity for use in chlor-alkali electrolytic cells without further primary treatment of the brine at the electrolytic cell plant installation site and has been found to be inoperative for that purpose, as will be explained.